Radios operating in the extremely high frequency (EHF) band of the electromagnetic (EM) spectrum exhibit numerous advantages, and are anticipated to play a significant role in communication technology—particularly wireless, mobile communication devices. For example, radios operating in EHF exhibit numerous advantages over radios operating in other frequency bands of the spectrum, including license-free spectrum, relatively narrow beam antennas, and inherent security due to oxygen absorption and the narrow beam width.
As used herein, the extremely high frequency (EHF) band of the EM spectrum includes frequencies from approximately 30 to 300 GHz. This is the highest frequency range of what is considered to be Radio Frequency (RF) EM radiation. Above this frequency band, EM radiation is considered to be in the low infrared light spectrum (also referred to as terahertz radiation). EM energy in the EHF band has a wavelength in the range of approximately 10 mm to 1 mm. Hence, EHF EM radiation is also generally referred to as millimeter wave RF (mm-wave). Accordingly, the terms EHF and mm-wave are used synonymously herein when referring to a frequency band.
In the U.S., the Federal Communication Commission (FCC) has allocated an unprecedented 7 GHz of un-channelized spectrum for license-free operation between 57-64 GHz. In contrast, less than 0.5 GHz of spectrum is allocated between 2-6 GHz for WiFi and other license-free applications. The portion of the EHF band near 60 GHz thus represents a significant opportunity to implement multi-gigabit RF communication links. Standardization efforts in this area include WiGig and WirelessHD.
EHF radios utilize very narrow RF beams, enabling multiple EHF radio base stations or other transceivers to be installed on the same tower, rooftop, or the like, even if they are all operating at the same transmit and receive frequencies. Co-located radios operating in the same transmit and receive frequency ranges can easily be isolated from one another based on small lateral or angular antenna separations, and/or the use of cross-polarized antennas. While the RF beams are relatively narrow, however, they are sufficiently wide, e.g., compared to optical signals, such that fixed antennas may be accurately aligned by a non-expert installer with the use of a simple visual alignment tool, and communications are unaffected by minor antenna movement such as tower or building sway due to wind.
Oxygen attenuates RF signals near 60 GHz (e.g., ˜57-64 GHz) due to a resonance of the oxygen molecule, a property that is unique to the near-60 GHz portion of the EM spectrum. While this property limits the distances that radio links at this frequency can cover, it also makes these links highly immune to interference from other radios at the same or near frequencies. For example, oxygen absorption ensures that a near-60 GHz signal will not extend far beyond its intended target.
The combination of narrow beam width and oxygen attenuation provides an inherent degree of security to near-60 GHz link communications. Due to the narrow beam width, an interceptor receiver must be placed directly in the main beam (and tuned to its carrier frequency) to receive a useful signal. In this position, it is likely to degrade the signal at the intended receiver sufficiently to allow for its detection. Due to oxygen attenuation, there is a limited distance beyond an intended receiver, along the main beam, at which a useful signal may be obtained by an interceptor receiver.
Accordingly, the demand is increasing for EHF capability in mobile communication devices, particularly near 60 GHz, to allow them to engage in communication channels supplemental to their primary channels (e.g., GSM, CDMA, LTE, and similar systems).
Since bandwidth is an expensive resource, most mm-wave transceivers make use of quadrature radio architectures so that both sides of the spectra can be used for information. Three common techniques used to generate quadrature signals are, (a) divide by two circuit along with an oscillator at twice the desired local oscillator (LO) frequency, (b) quadrature oscillators, and (c) single phase oscillator followed by polyphase filter (PPF). For more information, see the paper by A. Valero-Lopez, S. T. Moon, and E. Sanchez-Sinencio, titled, “Self-calibrated quadrature generator for WLAN multistandard frequency synthesizer,” published in the IEEE Journal of Solid-State Circuits, vol. 41, no. 5, pp. 1031-1041, May 2006, the disclosure of which is incorporated herein by reference in its entirety.
Option (a) has the drawback of requiring a signal at double the frequency of operation, and therefore low signal levels and high phase noise due to low quality passives at mm-wave frequencies. See the paper by W. Volkaerts, M. Steyaert, and P. Reynaert, titled, “118 GHz fundamental VCO with 7.8 tuning range in 65 nm CMOS,” published in the 2011 IEEE Radio Frequency Integrated Circuits Symposium (RFIC), pp. 1-4, June 2011, the disclosure of which is incorporated herein by reference in its entirety.
Option (b) generates the quadrature LO signal by two coupled oscillators which compromise the phase noise and tuning range. See the paper by K. Scheir, S. Bronckers, J. Borremans, P. Wambacq, and Y. Rolain, titled, “A 52 GHz phased-array receiver front-end in 90 nm digital CMOS,” published in the IEEE Journal of Solid-State Circuits, vol. 43, no. 12, pp. 2651-2659, December 2008, the disclosure of which is incorporated herein by reference in its entirety.
In option (c) a polyphase filter is used which can generally achieve wideband performance with sufficient quadrature accuracy by cascading two or more stages, which also simplifies the on-chip oscillator design compared to the other two techniques. See the papers by A. Parsa and B. Razavi, titled, “A new transceiver architecture for the 60-GHz band,” published in the IEEE Journal of Solid-State Circuits, vol. 44, no. 3, pp. 751-762, March 2009, and Notten, M. G. M. and Veenstra, H., titled, “60 GHz quadrature signal generation with a single phase VCO and polyphase filter in a 0.25 μm SiGe BiCMOS technology,” published at the IEEE Bipolar/BiCMOS Circuits and Technology Meeting, 2008, pp. 178-181, the disclosures of both of which are incorporated herein by reference in their entireties.
However, mm-wave frequency polyphase filter designs have not been analyzed extensively. Parasitic capacitance in mm-wave frequency polyphase filter designs utilizing conventional layout techniques lead to significant signal loss and hence operational inefficiencies.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.